Potassium ferro-heptanitroso sulphide, KFe₄(NO)₇S₃

To prepare Potassium ferro-heptanitroso sulphide, KFe₄(NO)₇S₃.H₂O, Pavel raised a solution of potassium nitrite to boiling, added sodium sulphide, and, finally, with repeated shaking, a solution of ferrous sulphate in small quantities at a time. Upon crystallisation the nitroso derivative separated out in monoclinic crystals which were purified by recrystallisation from warm water and carbon disulphide in succession, and finally dried over phosphorus pentoxide.

The same compound results when a dilute solution of potassium sulphide or sulphydrate is added to one of ferrous sulphate containing nitric oxide.

When a solution of potassium ferro-dinitroso thiosulphate, KFe(NO)₂S₂O₃.H₂O, is boiled, sulphur dioxide is expelled, and ferric hydroxide precipitated. Upon concentrating the clear solution obtained by filtering, crystals of potassium ferro-heptanitroso sulphide are obtained.

When acted upon with dilute sulphuric acid, the potassium salt yields the free acid, hydrogen ferro-heptanitroso sulphide, HFe₄(NО)₇S₃, as an insoluble, amorphous precipitate. It is very unstable. When boiled with dilute potassium hydroxide solution, the potassium hepta-nitroso derivative yields potassium ferro-dinitroso sulphide, KFe(NO)₂S.2H₂O.